A water level gauge is included in the pressure vessel of a nuclear reactor for measuring the water level therein. The output of the water level gauge is supplied to various control machinery and equipment such as the nuclear reactor protection system, the water level control machinery and equipment, and the water supply control system for maintaining the safety of the nuclear reactor. It is therefore required that the water level gauge indicates correct water level signals even in the case of a nuclear reactor accident.
The principle of the water level gauge of a conventional nuclear reactor is such that a differential pressure is measured reflecting a difference in hydrostatic pressure between the water head at the measuring point of the pressure vessel and a reference water head in a reference water level container communicating with the upper part of the pressure vessel by an upper part pressure tube. The water level is obtained by the following equation: ##EQU1## where h is the true water level inside the pressure vessel, .rho..sub.g is the density of the vapor inside the pressure vessel, .rho..sub.l is the density of the water inside the pressure vessel, H is the height of the reference water head, and .rho..sub.o is the density of the reference water head water.
However, the conventional water level gauge presents the following problems. During the normal operation of a nuclear reactor, the vapor inside the nuclear reactor is introduced from the upper part pressure tube to the inside of the reference water level container and is condensed therein so that the temperature of the reference water head water rises above 100.degree. C. to a depth of about 100 mm below the reference water level. Then, the density .rho..sub.o of the reference water head water is locally varied, so that measurements of the water level become incorrect and unstable. Further, in the case of leakage accident of the coolant due to a break in the main vapor tube of the nuclear reactor, an abrupt loss of pressure inside the pressure vessel results in boiling under reduced pressure of the water inside the reference water level container. Since the temperature inside the drywell of the housing reaches above about 170.degree. C. due to the leaked coolant, the reference water head water and the water inside the lower part pressure tube are boiled by this heat. Due to this, the output of the differential pressure detector for detecting the differential pressure is abnormally oscillated so that an erroneous signal is supplied to various pieces of control machinery and equipment.